Neutronic reactor



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United States arent "Ohce V2,910,418 Patented Oct.. 2'?, 1959 NEUTRONIC`REACTOR Edward C. Creutz, 'Leo A. lllinger, Alvin M.Wenberg, LEugene P.Wigner, Vand Gale J. Young, Chicago, Ill., assignorsto the United Statesof America as represented by the UnitedStates Atomic'Energy CommissionAApplicmnm January 2s, .1945, serial No. 574,153

1o claims. 4(ci. 2194-1932) "I'he present invention relates to thesubject 'of neutronics, and more particularly to a liquid cooled neutronchain reacting system, also referred to as a neutronic reactor, or pile,the latter name having'been originally vadopted for the lactive portionsof systems employing uranium or other ssionable bodies `geornetricallyarranged in` graphite or other moderator in the form of Morespecifically the invention has to do with liquid cooled neutronicreactors in which a coolant passes overbodies of iissionable material.disposed in channels inside the reactor and this coolant may bemaintained-under pressure which can readily be varied. VThe 'flow ofcoolant'through any one or more .of the tubes or channels containingfissionable material may be independentlyregulated and if desired may beentirely shut oif sothat a'defective tube or channel may "be"blocked offYand removed from the reactor vwithout permanently disabling theremainder of the reactor.

As aresult of the chain reaction, when U2.38 isrpresent '(as in naturaluranium), transuranic element 94239, known as plutonium, is produced.This material is svsionable and is valuable when added .to naturaluranium VH235,-l-neutron-aA-i-Bl-about 2 -neutrons (average) where Arepresents light fission 4fragments having atomicmasses ranging from 83to v99 inclusivevand atomic numbersfrom '34 'to 45 inclusive;forexample, Br, Kr, Rb, Sr, `Y, Zr, Cb, Mo, Ma, Ru,ar1d.Rh; and Brepresents heavy iission fragments .having atomic masses .ranging from`127 to :141 inclusive, and atomic numbers Vfrom 5'l`to 60 inclusive;for example, Sb, Te, I, Xe, Cs,

Ba, La, Ce, Pr, and Nd.

The elements 'resulting from the ssions are unstable land radioactive,with half-lives varying in length in accordance with the element formed.

The absorption of thermal or resonance neutrons by the U238 isotopegives rise-to the conversion of U238 to U239 which ultimately decays toVtransuranic lelement '94239. The reaction iseas follows:

`922394111 .92239 {plus 6 inev. of y rays, not neces- `2.3 days v 93239V94.239--[600 kv. upper energy limit. Also 2 'y rays, 400 kv. and 270kv., about of which are eonverted'to electrons] `Most ofthe-neutronsarisingfrom the ssion process are `set free -withthe very highI energyof above-one million electron volts average and are'therefore not .incondition to be utilized `efficiently to create newthermal neutronssions in afssionable' body such as U235 when it is y mixed with aconsiderable quantity of Um, particularly as Yin the case of naturaluranium. The energies of the fission-released neutrons are so high thatmost of the latter would tend to beabsorbe'd by the U238 nuclei, and yetthe energies are .notgenerally high enough for production of iission bymore 'than a small 'fraction of the neutrons so absorbed. VFor neutronsof thermal energies, however, lthe absorption cross section of `U235, toproduce fission, is a great deal more than the lsimple capture crosssection-of Um; so that underV the stated circumstances the fast 'fissionneutrons, after they are created, must be slowed downto `thermalenergies before they are most eifective to produce 'fresh ssion byreaction with additional "U235 atoms. If a system canlbe made in whichneutrons are slowed -down without excessive absorption Auntil they reachthermal energies and then mostly enter into uranium rather than into anyother element, `a self-sustaining nuclear chain yreaction can beobtained, even with Vnatural uranium. .Light elements, such asdeuterium, beryllium, oxygen or carbon, the latter in the :form ofgraphite, can .be used as slowing agents. A special advantage ofthe useofthe `light elements mentioned `vfor slowing-down fast ssion neutronsis that fewer collisions are required forslowing than is the case withheavier elements, and furthermore, the

' above-enumerated elements have very small neutron .cap-

ture probabilities, even for thermal neutrons. Hydrogen would bemostadvantageous were it not for thelfact that there may be a relativelyhigh probability .of .neutron capture `by' thehydrogen nucleus. Carbonin. the form of graphite is a relatively inexpensive, practical, and yreadily available-agent Vfor slowing fast neutrons to thermal energies.Recently, beryllium .has Y,been .made available in-suciently.largelquantities.for test as to suitability for usefas a.neutron slowing .material in aV system of the type tobe described. ItVhas been lfound to be in every way as satisfactory -as carbon.Deuterium while more expensive is especially valuable because of .its.low absorption of neutrons and its compounds such as deuterium oxidehave been used with very. effective results.

However, in order for the premise tobe fulfilled that the fast lissionneutrons `be slowed to thermal energies in a slowing medium withouttoolarge an absorption in the 11238 isotope'of the uranium, certain typesof physical structure should be utilized for the vmosteicient-reproduction ofV neutrons, since unless Vprecautions are takento reduce various neutron losses andf'thus to conserve neutrons for thechainreaction the rate-of neutron reproduction may be lowered andincertain cases lowered to a degree such that a self-sustaining systemis not .attained. Y

The ratio of the number of fast neutrons produced v.by

the ssions, to the original number of fast neutrons creating thefssions, in a system of infinite size using specific materials is calledthe reproduction or multiplication factor of the system and is denotedby the-symbol K. If K can be made sufficiently greater than unity tocreate a net gain in neutrons and the system made sufciently largesothat this gain is vnot entirely lost Yby leakage from the exteriorsurface of the system, then a selfsustaining chain `reacting system canbe built to produce power by nuclear iission of natural uranium. The.-neutron reproduction ratio, r, in a system of nite size differs from Kby the leakage factor and by localized neutron absorbers such as controlrods, and must be suiiciently greater than unity to permit the neutron.density to rise exponentially. Such a rise will continue indefinitelyif not controlled at a desired density corresponding to a desired poweroutput.

During the interchange of neutrons in a system comprising bodies ofuranium of any size in a slowing medium, neutrons may be lost in fourways, by absorption in the uranium metal or compound without producingfission, by absorption in the slowing down material, by

absorption in impurities present in the system, and by leakage from thesystem. These losses will be considered'in the order mentioned.

Natural uranium, particularly by reason of its U238 content, has anespecially strong absorbing power for neutrons when they have beenslowed down to moderate energies. The absorption in uranium at theseenergies is termed the uranium resonance absorption or capture. It iscaused by the isotope U238 and does not result in fission but createsthe isotope U239 which by two successive beta Vemissions forms therelatively stable nucleus 94239. It isv not to be confused withabsorption or capture of neutrons by impurities, referred to later.Neutron resonance absorption in uranium may take place either on thesurface of the uranium bodies, in which case the absorption is known assurface resonance absorption, or

it make take place further in the interior of the uranium body, in whichcase the'absorption is known as volume resonance absorption. It will beappreciated that this classification of resonance absorptions is merelya convenient characterization yof observed phenomena, and arises, notbecause the neutron absorbing power of a U238 nucleus is any greaterwhen the nucleus is at the surface of a body of metallic, or combineduranium, but because the absorbing power of U238 nuclei for neutrons ofcertain particular energies is inherently so high that practically allneutrons that already happen to have those energies, called resonanceenergies as explained above, are absorbed almost immediately upontheirarrival in the body of uranium metal or uranium compound, and thusin effect are absorbed at the surface of such body. Volume resonanceabsorption is due to the fact that some neutrons make collisions insidethe uranium body and may thus arrive at resonance energies therein.After successfully reaching thermal velocities, about 40 percent of theneutrons are also subject to capture by U238 without 'ssion, to produceU23-9 and eventually 94239.

It is possible, by proper physical arrangement of the materials, toreduce substantially uranium resonance absorption. By the use of lightelements as described above for slowing materials, a relatively largeincrement of energy loss is achieved in each collision and thereforefewer collisions are'required to slow the neutrons to thermal energies,thu's decreasing the probability of a neutron being at a resonanceenergy as it enters a uranium atom. During the slowing process, however,neutrons are diffusing kthrough the slowing medium over random paths anddistances so that the uranium is not only exposed to thermal neutronsbut also to neutrons of energies varying between the emission energy offission and thermal energy. Neutrons at uranium resonance energiesWill,'if they enter uranium at these energies,.be absorbed on thesurface of a uranium body whatever its size, giving rise to surfaceabsorption. Any substan- Vtial reduction of overall surface of the sameamount of uranium relative to the amount of slowing material (Le. theamount of slowing medium remaining unchanged) will reduce surfaceabsorption, and any such reduction in surface absorption will releaseneutrons to enter directly'into the chain reaction, i.e., will increasethe number of neutrons available for further slowing rand thus forreaction with U235 to produce fission.

For a given ratio of slowing material to uranium, sur'- face resonanceabsorption losses of neutrons in the uranium can be reduced by a largefactor from theV losses occurring in a mixture of fine uranium particlesand a slowing medium, if the uranium is aggregated into substantialmasses in which the mean radius of the aggregates is at least 0.25centimeter for natural uranium metal and when the mean spatial radius ofthe bodies isl at least 0.75 centimeter for the oxide of natural uranium(U02). Proportionate minimums exist for other uranium compounds theexact rninimumvalue being dependent upon the uranium content and thedenstiy of the product. An important gain is thus made in the number ofneutrons made directly available for the chain reaction. A similar gainis made when the uranium has more than the natural content of issionablematerial. Where a maximum K factor is to be desired we place the uraniumin the system in the form of spaced uranium masses or bodies ofsubstantial size, preferably either of metal, oxide, carbide, or othercompound or combinations thereof. The uranium bodies can be in the formof layers, rods or cylinders, cubes or spheres, or approximate shapes,dispersed throughout the graphite, preferably in some geometric pattern.The term geometric is used to mean any pattern or arrangement whereinthe uranium bodies are distributed in the graphiteor other moderatorwith at least either a roughly uniform v spacing or with a roughlysystematic non-uniform spacing, and are at least roughly uniform in sizeand shape or are systematic in variations of size or shape to produce avolume pattern conforming to a roughly symmetrical system. lf thepattern is a repeating or rather exactly regular one, a system embodyingit may be conveniently described as a'lattice structure. Optimumconditions are obtained with natural uranium by using a lattice of metalspheres.

The num-ber of neutrons made-directly available to the chain reaction byaggregating the uranium into separate bodies spaced through the slowingmedium is a critical factor in obtaining a self-sustaining chainVreaction utilizing natural uranium and graphite. The VK factor of amixture of [ine uranium particles in graphite, assuming both of them tobe' theoretically pure,r would only be about .785. Actual K factors ashigh as 1.07 have been obtained using aggregation of natural uranium inthe `best known geometry, and with as pure materials as it is presentlypossible to obtain.

Assuming theoretically pure carbon and theoretically pure naturaluranium metal, both of the highest obtainable densities, the maximumpossible K factor theoretically obtainable is about 1.1 when the uraniumis vaggregated with optimum geometry. Still higher K factors can beobtained by the use of aggregation in the case of uranium having morethan the naturally occurring content of ssionable elements. Adding suchlissionable material is termed enrichment of the uranium. Y

It is thus clearly apparent that the aggregation of the uranium intomasses separated in the slowing material is one of the most important,if not the most important factor entering into the successfulconstruction of a selfsustaining chain reaction system utilizingrelatively pure natural uranium in a slowing material such as graphitein the best geometry at present known, and is also important inobtaining high K factors when enrichment of the uranium is used.

Somewhat higher K factors are obtainable where moderators such asdeuterium oxide or beryllium are used.

Thus with beryllium it is-possible to secure a K factor as high as 1.10with optimum geometry and absolute purity. Moreover with deuterium oxideK factors of about -1.27 may be obtained. When'such moderators are usedthe problemrof aggregation may be somewhatr In addition totheabove-mentioned losses, which-are 4vir'rhelently a -partrof the nuclearchain reaction process, Vimpurities :presentginboth the slowing materialandthe uranium add a veryimportant Yneutron loss factor in the .ychain'The effectiveness of various elements as neutron absorbersvaries-tremendously. Certain elements such pas 4boron,cadmium, samarium4Agadoliniurn, and some tlothers, if-present even inalfewpparts-permillion, could v prevent aself-sustaining chain reactionfrom taking place. --It Iis highly-important, therefore, to :remove as4far as possible all impurities capturing neutrons to the detrirment of'the chain reaction from both the slowing material and the uranium. VIfthese impurities,solid, liquid, or

v gaseous, and -in elementalor combined form, are present in too greatquantity, in the uranium bodies or the slowling material or in, or byabsorption from, the free spaces ofthe system, ,the self-sustainingchain reaction cannot be attained. Thegamounts `of impurities 'that maybe permitted in a system, vary with afnumber of factors, such as thespeciiicgeometry ofthe system, andthe form in which the uranium isusedthat is, whether natural or enriched, whether as metal or oxideandalso factors such as the Weight ratios between the uranium and theslowing down material, and the type of slowing down or moderatingmaterial used-for example, whether deuterium, graphite or beryllium.Although all of these considerations influence the actual permissibleamount of each impurity material, it has fortunately been found that, ingeneral, the eiect of any given impurity or impurities can be correlateddirectly With the weight of Vthe impurity present and with the kK factor`of the system,

so thatknowing the K factor for a given geometry and fcomposition, thepermissible amounts of particular impurities can be readily computedwithout taking individual account of the vspecific considerations namedabove. 'Different impurities are found to aifect the operation toVwidely diiferent extents; for example, relatively considerablequantities of elements such as hydrogen Ymay be r tivebarometer is thusobtained. In general,the inclusion of combined nitrogen is to beavoided.

The eifect of impurities on the` optimum reproduction factor K may beconveniently evaluated to a good approx-V imation, simply by means ofcertain constants known as danger coeicients which are assigned to thevarious elements. These danger coeiiicients for the impurities are eachmultiplied by the percent by weight of the correspondlng impurity, andthetotal sum of these products gives a value known as thegt'otal dangersum. This total danger sum is subtracted from the reproduction factor'Kas calculated for pure materials and for the specific` geometry underconsideration.

The danger coellicients arefdened in tenns ofthe ratio of the weight ofimpurity per unit mass of uranium and are based on the cross section forabsorption of thermal neutrons of `the various elements. These valuesmay be 65 obtained from physics textbooks on the subject and the dangercoeiiicient computed by the formula nu Ai wherein ai -representsf thecross section for the impurity -and ru the cross section for'theuranium, Ai the atomic weight of the impurity and A11 the atomicweight'for uranium. lfthe impurities are in the carbon, they arecomputed as theirpercent ofthe weight of the-,uranium iof -the system.

rPresently .known Avalues ffor ;,danger,;coe1iicients for Vsome elements'are given 'gin `the following table, wherein the elements are assumedto have.their-naturalfisotopic constitution unless otherwise indicated,and 4are-c011- veniently .listed according to their chemical fsymbols:

, Danger Where aan element is fnecessarily zused `in an active part. ofYa system,.it is stillto be considered as anirnpurity; for example, -in-astructure Where the uranium bodies consist of uranium oxide, theactual K factor would ordinarily be computed byltaking that factintoaccount usingfas a base K a Value computed for theoretically v pureuranium.

As `a specicexample, if Vthe materials offthe system Vunderconsideration have .0001 part b-y weight of Coand Ag, thetotal 'dangersum in K-.units "for'such an analysis 4would be:

.o001 1+.0001 1s=.o035Kunits This -would be a lrather unimportantreduction in the re- `pro iuctionfactor K unless the reproduction-factor fora given system, .withoutconsidering any impurities, is-verynearly unity. If, on the Vother hand, the impurities Vin the uranium inthe previous example had :been Li, Co, and Rh, the total danger sumwould be:

' This latter reduction Vinthe reproduction factor for a given systemwould be serious and might well reduce the reproduction factor belowunity for certain geometries and certain moderators so as to make isimpossible to eiect a self-sustaining chain reaction with naturaluranium and graphite, but might still be permissible when using enriched.uranium in a system having a high K factor. Y

' This strong absoring action of some elements renders Vaself-sustaining chain reacting system capable of con trol. Byintroducing neutron absorbing elements inthe form.- of rods or sheetsinto the interior of the system, for instance in the slowing materialbetween the uranium masses, the neutron reproduction ratio of the systemcan be changed in accordance with the amount of absorbh ing materialexposed to the neutrons in the system. A sufficient mass of theab-sorbing material can readily be inserted into the system to reducethe reproduction ratio of the system to less than unity and thus stopthe reaction. Consequently, it is another object of our invention toprovide a means and method of controllingrthe cham reaction in aself-sustaining system. Y l

When the uranium and the slowlng materlal are of reproduction factor Kis greater than unity, the numberl of neutrons present will increaseexponentially and indefinitely, provided the structure is madesufficiently large. If, on the contrary, the structure is small, with alarge surface-to-volume ratio, there will be a rate of loss of neutronsfrom the structure by leakage through the outer surfaces, which mayoverbalance the rate of neutron production inside the structure so thata chain reaction will not be self-sustaining. For each value of thereproduction factor K greater than unity, there is thus a minimumoverall size of a given structure known as the critical size, abovewhich the rate of loss of neutrons by dilusion to the walls of thestructure and leakage away from the structure is less than the rate ofproduction of neutrons within the system, thus making the chain reactionself-sustaining. The rate of diffusion of neutrons away from a largestructure in which they are being created through the exterior surfacethereof may be treated by mathematical analysis when the value of K andcertain other constants are known, as the ratio lof the exterior surfaceto the volume becomes less as the structure is enlarged.

In the ca se of a spherical structure employing uranium bodies imbeddedin graphite in the geometries disclosed where a, if, and c are thelengths of the sides in feet. The

Ycritical size `for a cylindrical structure is giveny by the formula,irrespective of the shape of the uranium bodies, cylinder height h ft.,radius R ft.

However, when critical size is attained, by definition no rise inneutron density can be expected. It is therefore necessary to increasethe size of the structure beyond' the critical size but not to theextent that the period for doubling of the neutron density is too short,as will be explained later. -Reactors having a reproduction ratio (r)for an operating structure with all control absorbers removed and at thetemperature of operation up to aboutl 1.005 are very easy tofcontrol.Reproduction ratio should not be permitted to rise above about 1.01since the. reactionV will become ditlcult to control. The size Vatwhichthis reproduction ratio can be obtained may be computed frommodifications of the above formulae for crltical size. For example, forspherical active structures the formula maybe used to find R when K isknown and r is somewhat over unity. The same formula will, of course,give r for given structures for which Kl and R are known.

' Critical size may be attained with a somewhat smaller structure byutilizing a neutron reflecting medium surrounding the surface of theactive structure. For example, a 2 foot thickness of graphite having lowimpurity content, completely surrounding a'spherical structure-iselfective in reducing the diameter of the uranium bearing portion byalmost 2 feet,rresulting in al considerable saving of uranium or uraniumcompound.

The rate of production of element 94239 will depend on the rate ofneutron absorption by U238 and is also proportional to the rate at whichfissions occur in U235. This in turn is controlled by the thermalneutron density existing in the reactor While operating. Thus formaximum production of element 94239, it is essential that the thermalneutron density be at a maximum Value commensurate with thermalequilibrium.Y

Considerable heat is generated during a neutronic reaction primarily asthe result of the fission process. Following are tables showing morespecifically the type of heat generated in the reactor.

SUMMARY BY TYPE Mev./ Percent fission Gamma. radiation Y 18 9 Betaradiation 16 8 Kinetic energy of ssion fragment 160 80 Kinetic energy ofneutrons 6 3 SUMMARY BY LOCALE WHERE HEAT IS GENERATED Mev./ Percentfission In uranium 174 87 In moderator..l v 16 8 Outside pile 10 l l 5SUMMARY BY TYPE AND LOCALE Mev. per Percent Percent Percent fission in Uin C Outside Kinetic energy of fission fragments 159 100 Kinetic energyof neutrons 6 99 1 Gamma radiation from fission products 5 50 45 5 Betaradiation from ssion products 6 100 Nuclear aflinity of neutrons (gamma,radiation) 12 70 25 5 When the system is operated for an extended periodof time at a high production output of element 94239, the

large'amount of heat thus generated must be removed in order tostabilize the chain reaction. Most of the heat in an operating device isgenerated as the result of the nuclear ssions taking place in the U235isotope. Thus, the rate of heat generation is largely proportional tothe rate at which the fissions Ytake place. In other words, if the rateof generation of neutrons is increased, a greater amount of coolant mustbe passed through the reactor in order to remove the heat thus generatedvto avoid damage, particularly at the central portion of the pile, byexcessive heat. Thus, the highest obtainable neutron density at which asystem can be operated for an extended period of time is limited by therate at which the generated heat can be removed. That is to say, themaximum` power output of a system is limited by the capacity of thecooling system. An eiective cooling system is therefore a primaryrequirement for high power operation of a neutronic reactor and it hasbeen found time sucient to'cause Va quantity-Lof l{elementx942-39 to "berproduced, it may be desirable` to .removeatleast some v.ofVthe-uranitun rods `from the.reactor 4in order to extract element-9433-9and the-radioactiveziission products, fboth being formed intheura'niumrods oriforother purposes.

fIn ma-ny neutronic reactors, .a aneutron density Avariationoccufrs-across `the reactor; that is, Vthe :neutron .ccn- -centration-atthefperiphery -is relatively ,small .and jincreases to-.a maximum .valueat'the :centen` Actually, ytther =.fore,-since the Yrate Aofi-production`of element `943-39 is dependent rupon the .neutron density, -tthereactor, will -have zones -whichrnay be likened to three-.dimensionalshells, 'the `average concentration.oflelement 94239 being uniformthroughout any given zone. In 4areactor.built vin the form of aspheretthesewould,ofrcoursepberin the .-shapevoffconcentric spheresofdifferentadiameters, .while one `.built Ain rthe shape of. a cylinder.would have ,similar zones but odifierentfshapes.

A-Where "this variation in concentration exists in.a re- -aetoritisfoften desirable toresort toy arsysternaticV schedulerofremovaldepending V:upon Athetirne of ,operation :and the `location of:the -uram'um foriremoving and discharging uranium metal that has vbeensubjected to meut-ron bombardment.` :in the4 caseof ainew Lsystem lof.-this char- -acter the operation .would .normally .continue until :the-metallin-thecenter portionfof thereactorreaches a.desired content ofelement 942,39, at Vwhich time this metal would be Vremoved landreplaced-with ifresh. metal. The next removal -thenwould tbe .from :thesection vnextl adjacent to the centerlsection of .the freactorlwhere thedesired content of element 949.39 -.is reached after further Loperation.The process would then .proceed '.Withuthe removal lofthe metal -atvarious times vuntilithe metal :recharged at the center of the reactoryhas .reached .thedesiredontentofelement 94239. This -wouldthen abereplacedand the process of progressing 4towards the periphery 'continuedwith periodic return-tormore central areas. Since the neutron densityin'the centralareas of snch a-reactor would, ordinarily, greatly exceedthe neutron density 'near the'periphery, the metal .in the central areas.may be replaced .several times foreach -replacemennof the zmetal nearthe periphery. A removal schedule can be deivelopedJbycalculation andcheckedby 4actual experience afterfthe system, has-been .placed.inoperationL Different schedules .may :be developed with other reactorshaving dierent reactivity curves. For exampleycer-` tainreactors-'areconstructed in amanner such that the i tope; andarlolonger. change. The 94239 on ;the other hand v.is a relativelystableelementwh'en 4forn1ed,having aradio- .fmetal subjected ato intenseneutron Vbombardment the -(radioactive jiission .elementswill-,reachastate of `equilirbrium .andzfrom-.that time on theamounts-of theseradioactiveelernentsremain constant, vas the fission elements .-wlth.shorterhalfflives are reaching .a .stable condition Aat the A,same-timenew onesarebeing produced. i The amount ofthe stable end .products ofiission, however, continues @to increase .with the increase Vin element942.39.

Consequently, the rate:oftformation of the fission end products 1sdependent upon the location of any particular :metal V.in the reactor,andthe power at which the system operates controls themaximumradioactive.iission elementcontent 4regardless of fthelength oftime the system Yoperates vafter equilibrium occurs. The quantity ofelement 94239 on :the other hand, and of theiinal and stable endproducts of fission continue `to-increase as the operation of the systemcontinues. 'The amounts of both 94239 and `lission endproducts presentare controlledonly by the location of the metal in -the-reactorand thetime and fpowerof operation. The-'highly radioactivelission elementsmay,

- therefore,` v-ary from a substantial 4percentage of theweight -ofelement' 9423.9 'present in the Ymetal at -thef center -of Ythe reactorVafter a short period of operation, to-a' very `smallpercentage Vinmetal from a'position near-the--pe- "iission elements have beenpermitted to decay-for atime 1equal to the ,equilibrium period, ,forexample. Many of theoriginal ssione'lements have longhalf lives that,

. taken together ,Withjtheir successive radioactivedistintengrationproducts existing long after the fission elements"jhaving a shorten-half Vlife vhave decayed, ,renders the 1 uraniumstill radioactive ,especiallyafter .prolonged bombardment athigh neutrondensities. In addition, the successiverradioactive disintegrationproducts of the original .Shorter lived iissionelementsrnay still bepresent.

The equilibrium radioactivity is so intense that metal f tal'ren'fromthe ,reactor for the recovery of element 94239 -neutron concentration-isfsubstantiallyuniform'` throughout a large volume of the reactor. Insuch a casethe schedule for r'ernoval'bf uranium bodiesV may be`modified accordingly.

Since `the heat 'generated in the reactor results from -issions in theuranium, itis evident that -t-hisheat Ais not 'formed uniformlythroughout -the -reactor but-that lit -must vary across the reactor withthe local rate-at vwhichiissions occur and element 94299 formed..'Consequently,.the

Y `relative values lfor the `production iof elementf'94239 .applyelements arrive at a stable non-radioactive element or isof and fissionproducts vimmediately after bombardment at high neutronV densities willheat spontaneously due to .reach this-concentration; and 3) the elapsed,time since sthe reactor was shut down and themetalwas removed. ,1;

' The metal ,from the ,center ofthe reactorgin a system opv erating atahigh power output, for example, at a v94239 ,QOnQentration of l ftoV2,000, if not cooled, can 'increase in temperature at the rate-of about2000 C. per hourone Iday after the neutronactivity of the system hasbeen shut down. -AfterSO days. shut down .following anoperation of 100days atanoutputof 500,000 kilowatts, the average Vtemperature rise canbe approximately 572 C. per hour. 'Ihe uraniummetal of -the type Vusedin the ,chain reacting systems herein under consideration melts at aboutl,l00 C. Y l

Under these conditions uranium bombarded with neui trons for an extendedperiod oftime at high rates of power (1) The neutron-activity ofthe'systemfis-shutdown` 11 and the uranium is kept in the reactor andcontinuously cooled until the radioactivity decays to a point where themetal can be removed without melting in ambient air. This procedure mayrequire that the metal remain in the reactor for a periodof from 30 to50 days after the` -neutron bombardment has ceased.

(2) The neutron activity of the-system is shut down and the uranium iskept in the reactor with the cooling system in operation for only a fewIdays to permit the most violent radioactivity to subside and thenthemetal is removed from the reactor with the cooling discontinuedduring the removal except for cooling by the atmosphere or vby waterspray. The metal is then promptly placed under more efficient coolingconditions before the temperature of the uranium has become excessive.

(3) The neutron activity of the system is shut down and the uranium isremoved while cooling the uranium body at least to an extent sufficientto prevent the temperature from becoming excessive. This modiiication ofthe present invention is particularly effective.

It is also important, of course, from the point of view of biologicalsafety of operating personnel that adequate shielding be provided toabsorb the strong gamma radiations from the fissionv products present inthe active uranium while being removed from the reactor, The neutronactivity in the reactor completely ceases Within 30 minutes after shutdown of the neutronic vreaction during which period delayed neutrons arebeing emitted. In no case then should the uranium be removed from thereactor immediately following shut down of the neutronic reaction, butsufcient time should be given to permit all delayed neutrons to beomitted. Thus, the shielding required during the removal of the uraniumrods from the system is primarily intended to protect personnel fromgamma radiations. As stated above, immediately following shut down ofthe neutronic reaction, there are many short lived radioactive fissionelements in the uranium causing the gamma radiation to be very intense.'Many of these elements decay into more stable products Within the rstthirty minutes following shut down of theV reaction. Thus, the issionproducts lose a largeamount of their radioactivity during this period.

While the method of extracting the iission products i and element 94239from the bombarded uranium taken from the reactor forms no part of thepresent invention, the ssion products and element 94239 are removableand when removed are extremely useful. The radioactive fission productsare valuable for use as radiation sources, many having long half liveswith high energy gamma radiation sufiicient for radiography of evenheavy metal castings. In addition, some of the fission products areuseful as radioactive tracers in biological and physiological research.f

Element 94239 is exceptionally usefulV because it is fissionable by slowneutrons in the same manner as the uranium isotope 92235 contained innatural uranium. The separation of 92235 from 92233 in natural uraniumis extremely difficult since both are isotopes of the Isame element andthese isotopes vary only a small percentage in comparative weight.Element 94239 on the other hand, is a different element from uranium,having diierent chemical properties than uranium, and therefore can bechemically separated from uranium. After separation,

for example, element 94239 can be added to natural uranium tosupplement4the 92235 content, `thus increasing the amount of fissionable materialin the uranium.

This enriched uranium can then be used in neutronic systems making itpossible to provide more cooling facilities, for example, than-can beused in asystem of the same geometry employing only natural'uranium.Thus, an enriched neutronic system may provide 4a greater power outputthan would be possible in a natural uranium system having the samegeometry.

To summarize, the present invention is concerned with a liquid cooledneutronic reactor capable of generating large quantities of heat and ofproducing element 94239 andy radioactive fission products and is welladapted for long and continuous operation. The pressure of the coolantflowing through -the reactor can be regulated either generally or byzones inside the reactor and any one or more portions of the reactor maybe blocked off without in any way disabling the entire reactor.

Other' utilities of neutronic reactors are disclosed in assigneescopending application, Serial No. 568,904, filed on December 19, 1944,by Enrico Fermi and Leo Szilard, now Patent No. 2,708,656,` dated May17, 1955, which discloses and claims neutronic reactors wherein surfaceresonance absorption losses 4of neutrons in the uranium are. minimizedby aggregating uranium bodies into substantial masses and by appropriatespacing as described above.

The foregoing constitute some of the principal objects and advantages ofthe present invention, others of which will become apparent from thefollowing description read in conjunction with the drawings, in which:

Fig. l is a diagrammatic view of a preferred embodiment of thepresentinvention showing horizontally disposed tubes in a graphitemoderator and further illustrating a water filled chute into whichuranium rods are ejected; I

Fig. 2 is a diagrammatic view of a second embodiment of the presentinvention similar to'Fig.v 1 but showing the tubes disposed verticallyin the graphite moderator; Y

Fig. 3 -is a diagrammatic view of a third embodiment of the presentinvention showing tubes arranged horizontally in .a graphite moderatorand further illustrating shielded cars for chargingl and discharginguranium rods into and from the reactor;

Fig 4 is` a schematic diagram showing the external circulating systemfor the coolant;

Fig. 5 is a plan view of the power unit forming the preferred embodimentof the invention;

Fig. 6 is a vertical sectional view taken on the line 6-6 of Fig. 5, theview being shown partially in elevation;

Fig. 7 is a vertical sectional view taken on the line f7-#7 of Fig. 6,ythe view being shown partially in elevation; Y

Fig. 8 is a vertical sectional view taken on the line vvation;

Fig. 9 is an enlarged fragmentary front elevational Vview showing theends of the tubes at the loading end of the reactor; v

Fig. 10 is an enlarged fragmentary longitudinal sectional view takenthrough two segments of uranium rods showingthe interlock between rodsegments;

Fig. 11 is an enlarged fragmentary view at the loading end of the`reactor showing partially in section and partially in elevation, therelationship between the uranium rod loading car and the horizontallydisposed Vtubes in the reactor during the loading `of uranium rods intothe reactor; l l

Fig. 12 isan enlarged fragmentary detailed sectional view correspondingto Fig. l1 but showing in particular the valve arrangement at theloading end of the reactor;

'Fig 13 is an enlarged transverse sectional view through one of 'thehorizontal tubes in the reactor showgraphite moderator'surrounding thetube;

Fig. 14 is a fragmentary vertical view through the reactor shownpartially in section and partially in elevation; Y

1. IN A NEUTRONIC REACTOR, A NEUTRON MODERATOR CAPABLE OF SLOWING FASTNEUTRONS TO THERMAL ENERGY, A TUBE THROUGH THE MODERATOR, A ROD IN THETUBE AND CONTAINING A THERMAL NEUTRON FISSIONABLE MATERIAL, THE RODHAVING AN AREA IN CROSS SECTION LESS THAN THAT OF THE TUBE, LONGITUDINALRIBS BETWEEN THE ROD AND THE TUBE WALL FORMING PASSAGES DISPOSEDLENGTHWISE WITH RESPECT TO THE TUBE, AND A COOLANT IN THE PASSAGES,SUFFICIENT OF THERMAL NEUTRON FISSIONABLE MATERIAL BEING USED NEUTRONICREACTOR TO PROVIDE A NEUTRON REPRODUCTION RATIO GREATER THAN UNITY.